In the United States alone, there are estimated to be about 3 million underground petroleum storage tanks, about 1 million in gasoline service and about 2 million in fuel oil service. In addition to the tanks in gasoline and fuel oil service, there are an unknown number of underground storage tanks storing other petroleum materials. It is estimated that about 31% of the total underground storage tank population are 20 years old or older. Consequently, the risk of corrosion failure of such a large number of tanks is significant. Undetected leaks, of course, present not only an economic loss, but also a threat to the environment. Thus, it is essential that such leaks be detected as soon as possible so that the tanks can be replaced or repaired. Indeed, current industry standards require that any detector employed in tank leak testing be capable of detecting leaks of the order of 0.05 gallons per hour. This capability is not easily or inexpensively achieved. As is known in the art, the temperature of a fluid in an underground tank, for example, can change continually at a rate of up to about 0.01.degree. F. per hour. Temperature changes of this magnitude will cause volume changes of up to 0.06 gallons per hour in a 10,000 gallon gasoline tank. Consequently, a slight change in temperature of the tank fluid produces a volume change which is greater than the amount of leakage which is sought to be detected. Therefore, leak detecting systems employ elaborate temperature measurement systems in addition to tank content level measurement systems. In that regard, see, for example, U.S. Pat. No. 3,538,746; U.S. Pat. No. 3,580,055; U.S. Pat. No. 3,841,146 and U.S. Pat. No. Re. 31,884, each of which describe leak detectors designed to measure and/or compensate for temperature changes of tank fluids. The deficiencies of each of these detectors should be readily apparent.
The detector of U.S. Pat. No. 3,538,746 is based on measuring the difference in pressure between the tank fluid and a filled reference leg in a partially filled tank. Presently available differential pressure sensors capable of being inserted through a tank fill pipe, however, are not sufficiently accurate to detect leaks of 0.05 gals/hr. within a reasonable time. Additionally evaporation and condensation effects will mask this systems leak detection capabilities.
The detector of U.S. Pat. No. 3,580,055 requires circulating fluid in the tank to an external temperature sensor in an attempt to thermally equilibrate the liquid and make a temperature correction in determining whether a leak exists. The need for a skilled operator is particularly important with this device. Raising the liquid hydrostatic head above ground level causes significant tank bulging which can mask the volume change due to a leak.
The devices of U.S. Pat. No. 3,841,146 and U.S. Pat. No. Re. 31,884 require elaborate and relatively expensive electronic circuitry in order to measure weight or mass changes to determine tank leakage.
Thus there remains a need for a new and improved tank leak detector which is sufficiently sensitive so as to be able to accurately detect leaks of the order of 0.05 gallons per hour and less in underground tanks having a volume, for example, of from 5,000 to 10,000 gallons or more and which is relatively inexpensive and simple to operate.